This invention relates to sorption bed systems of the type in which a mass of sorbent material is moved relative to a series of fluid streams such that a first section of the sorbent mass is in sorption duty while another section is being regenerated. More particularly, the invention relates to the design and operation of such beds so as to exploit to advantage the physical chemistry of sorption, desorption, heating, and cooling disclosed in U.S. Pat. No. 4,324,564, and to significantly improve the efficiency of the sorption-regeneration cycle in such beds.
Conventional rotary sorption bed systems utilize active sorbents such as lithium chloride immobilized on a porous support, alumina, zeolites, and other adsorbent or absorbent materials. They are used in situations where it is desired to separate a sorbable component from a fluid stream. Where the goal is to produce fluid of a reduced sorbate content, e.g., to produce dehumidified air or substantially water-free natural gas, fluid product exiting the sorption section is collected and the waste exiting the regeneration zone is discarded. In other situations, e.g., hydrocarbon recovery from an air stream, the "product" is discarded and the "waste" is collected.
Known recirculating beds may be classified as two or three stage devices. In a two-stage device, one portion of the bed mass is being loaded as a fluid feed passes therethrough in a sorption region and a second portion is simultaneously being regenerated with hot gas. As regeneration is completed, the hot, regenerated portion of the bed mass moves back into the sorption region while still hot. Product exiting from the bed in the first part of the sorption region cools the sorbent and is itself heated. In three stage devices, a cooling region is interposed between the end of the regeneration region and the beginning of the sorption region. Thus, cooling fluid passes through the hot, regenerated portion of the bed and drives out the heat, which typically is discarded as low grade waste heat, and cools the sorbent material prior to the beginning of its sorption duty.
During regeneration of conventional thermal swing sorbent beds, there is created in the bed a front, designated herein as the "RW" front, which is bounded on its downstream side by bed conditions characteristic of equilibrium between the sorbent material and fluid waste ("W", sorbate-rich effluent), and on its upstream side by bed conditions characteristic of equilibrium between the sorbent material and the hot regenerant fluid ("R"). After the regenerant has heated and dried the bed, and the RW front has exited the bed, a cooling fluid is introduced. Upon the introduction of the cooling fluid (here, for simplicity, assumed to be cool product, P), there is also created a wave or front, designated herein as a thermal front, which moves more rapidly than the RW front. The thermal front can arise in several ways. When regeneration is conducted using sufficiently hot fluid containing a substantial concentration of sorbate and the coolant is sorbate-free, a "PR" transition is created comprising a fast stripping front which removes all or most of the sorbate in equilibrium with the hot regenerant, and a thermal front which effects the major amount of temperature transition. Upstream of this transition from equilibrium with coolant to equilibrium with regenerant, the bed is in equilibrium with coolant (P). Downstream of the transition the bed is in equilibrium with regenerant ("R"). Other situations, where sorbate is present in the regenerant and/or the coolant, produce multiple fronts which together form the PR transition, one of which comprises the major thermal front. When regeneration and cooling are accomplished using substantially sorbate-free fluid, the PR transition is a pure thermal wave which is bounded on its downstream side by bed conditions characteristic of equilibrium between the sorbent and the hot regenerant fluid, and on its upstream side by bed conditions characteristic of equilibrium between the sorbent and the cooling fluid.
U.S. Pat. No. 4,324,564 discloses a novel method of operating sorption beds (the "Four Front System") and apparatus for practicing the method. In accordance with the teachings of the patent, it is possible to significantly reduce the quantity of heat needed for regeneration, to increase the throughput of a bed of a given size, to decrease the size and thus the capital costs of a bed system required to achieve a given capacity, to upgrade the quality of the product (reduce sorbate concentration), and/or to provide greater security against breakthrough of feed during the sorption stage.
The patent discloses that the cooling fluid may be introduced into the bed prior to the breakthrough of the midpoint of the RW front through the bed exit to achieve one or several of the foregoing operational advantages. Preferably, introduction of the cooling fluid is timed such that the thermal component of the PR transition is present in the last third of the bed length or most preferably at the fluid exit when the slower RW front is at or breaking through the fluid exit. Operation of the bed in this manner can result in a waste fluid whose temperature as measured at the bed exit never attains the temperature of the hot regenerant. As one result, significantly less heat is required to regenerate the bed.
In the practice of the Four Front Method, the precise timing of the beginning and end of the sorption stage and the hot regenerant and cooling fluid steps of the regeneration stage for a given system depends primarily on bed length, the particular sorbate-sorbent pair involved, fluid flow rates, and a balancing of the importance of the desired product characteristics, the reduction in heat consumption, the desired degree of security against breakthrough, and the maximization of throughput.